1. Field of the Invention
The present invention is related to a method for manufacturing a magnetoresistive (MR) effect element and to the MR effect element manufactured by the method. Also, the present invention is related to a method for forming a zinc oxide (ZnO) film.
2. Description of the Related Art
Zinc oxide (ZnO) is an important material that is used in various fields. For example, ZnO has been widely used in cosmetics to take advantage of its ultraviolet ray absorbing function, as a vulcanization accelerator of rubber, and in electric components such as a gas sensor and a varistor. In recent years, it has attracted attention as a transparent electrode film as an alternative to Sn doping In2O3 (ITO). As a transparent electrode film, ZnO has been already used for a white light emitting diode, a touch panel, a thin-type display device and a solar battery, and its use has been rapidly increasing. Furthermore, it is discussed that ZnO has a potential as an alternative to expensive GaN-system materials as an illuminator of a blue/violet light emitting diode.
It is known that ZnO, which is widely used as functional material as described above, can be used as a nonmagnetic intermediate layer of an MR effect element (for example, see Japanese laid-open application No. 2008-91842). Hereafter, the thin-film head technology using the ZnO film for the nonmagnetic intermediate layer is described.
As the density of hard disk drives increases, the performance of thin film magnetic heads is required to be improved. As a thin film head, a composite-type thin film magnetic head is widely used. In the magnetic head, an MR effect element for reading and an induction electromagnetic conversion element for writing are layered.
Generally, for an MR effect element, a shield layer is built for eliminating effects of magnetic fields from any parts other than target bits for reading in a recording medium as much as possible. The current mainstream technology is a current perpendicular to the plane (CPP) structure in which a pair of shield layers and an MR effect element are connected electrically in series via a metal film. It is said that the CPP structure will inevitably achieve a recording density of 500 Gbits/in2 or more.
The MR effect element of the CPP structure includes a pair of magnetic layers and a nonmagnetic intermediate layer sandwiched between the pair of magnetic layers. The MR effect element is configured to run a sense current in a direction perpendicular to film surfaces of the pair of magnetic layers and the nonmagnetic intermediate layer. One of the pair of magnetic layers is a magnetic free layer whose magnetizing direction can be changed according to an external magnetic field, and the other is a magnetic pinned layer whose magnetizing direction is fixed to the external magnetic field. In a situation without an external magnetic field, the magnetizing direction of the magnetic free layer is orthogonal in the magnetizing direction of a magnetic pinned layer by a bias magnetic field; however, the magnetizing direction of the magnetizing free layer rotates when an external magnetic field is applied. As the result, corresponding to a relative angle of the magnetizing direction, spin-dependent scattering of conduction electrons forming the sense current varies, and a change of magnetoresistance occurs. This change of the magnetoresistance is detected, and magnetic information of the recording medium is read.
Since the shield layers and the MR effect element are connected via metal films, the CPP structure has large radiation efficiency, and since the CPP structure structurally has low resistance, a large sense current can flow. These features indicate that the CPP structure is suitable for increasing resistant value and resistance change volume by decreasing its cross section. Namely, the MR effect element of the CPP structure has an advantage that is suitable for narrowing track width.
However, the MR effect element of the conventional CPP structure has merely a small magnetoresistive change rate (MR rate) so that the output is insufficient for practical use. Specifically, in practical use of a head of 300 Gbpsi or more, it becomes difficult to maintain a signal/noise (S/N) ratio when the MR rate is small. One reason that the MR rate of the MR effect element is small is that small electric resistance copper (Cu) is used for the nonmagnetic intermediate layer. Therefore, it is discussed that ZnO, which is a semiconductor as mentioned above, is used for the nonmagnetic layer. It is disclosed in Japanese laid-open application No. 2008-91842 that an MR effect element using ZnO for the nonmagnetic intermediate layer shows a significantly high MR rate.
However, there is a drawback in the MR effect element using ZnO for the nonmagnetic intermediate layer. The drawback is a large diversion of element resistances and a MR rate. When the diversion is large, a yield of the magnetic head that meets a required output performance is deteriorated, resulting in low manufacturing efficiency.
An object of the present invention is to provide the MR effect element that uses ZnO for the nonmagnetic intermediate layer and that suppresses diversions of an element resistance and MR rate, and a manufacturing method for the same. Further, the other object of the present invention is to provide a manufacturing method of a ZnO film that is useful for manufacturing the MR effect element and that is applicable to other technical fields.